Clean Enough for Industry?
an Airborne Geophysical Case Study

ABSTRACT

Data from two airborne geophysical surveys of the Department of
Energy's Oak Ridge Reservation (ORR) were extremely valuable in
deciding whether a 1000-acre (400 hectare) parcel of the ORR should
be leased to the City of Oak Ridge for industrial development.
Our findings, based on electromagnetic and magnetic data, were
incorporated in the federally mandated Environmental Assessment
Statement (EAS), and in general supported claims that this land
was never used as a hazardous waste disposal site. We estimated
the amount of iron required to produce each anomaly using a simple
dipole model. All anomalies with equivalent sources greater than
approximately 1000 kg of iron were checked in the field, and the
source of all but one identified as either a bridge, reinforced
concrete debris, or a similarly benign object. Additionally, some
smaller anomalies (equivalent sources of roughly 500 kg) have
been checked; thus far, these also have innocuous sources. Airborne
video proved invaluable in identifying logging equipment as the
source of some of these anomalies. Geologic noise may account
for some of the remaining anomalies. Naturally occurring accumulations
of magnetic minerals in the soil on the ORR have been shown to
produce anomalies which, at a sensor height of 30 m, are comparable
to the anomaly produced by about 500 kg of iron. By comparison,
the electronic noise of the magnetic gradiometer, 0.01-0.02 nT/m,
is equivalent to only about 50-100 kg of iron at a 30 m sensor
height. The electromagnetic data, combined with field mapping
of karst structures, provided evidence of a northeast-southwest
striking conduit spanning the parcel. The possible existence of
a karst conduit led the EAS authors to conclude that this is a
"sensitive hydrologic setting." We conclude that aerial
geophysics is an extremely cost-effective, and efficient technique
for screening large tracts of land for environmental characterization.

INTRODUCTION

The proposed action evaluated in this environmental assessment
(EA) is the lease of approximately 1000 acres of the U.S. Department
of Energy (DOE) land on the Oak Ridge (Tennessee) Reservation
(ORR) to the East Tennessee Economic council (ETEC). Under the
authority granted in Section 3154 of the Defense Authorization
Act of 1994 (Public Law 103-160), the Secretary of Energy may
lease real property at a DOE facility that is (1) to be closed
or reconfigured, (2) not currently needed by DOE, and (3) under
DOE control. Per the terms of the proposed lease, the sole purpose
for which the lessee (ETEC) would use the parcel of land (designated
Parcel ED-1) would be to further and support economic development
in the region. The need for DOE action is driven by a decreased
budget that will impact jobs and the economy on Oak Ridge and
surrounding communities. The proposed action is wholly intended
to help offset potential economic losses resulting from DOE and
contractor downsizing in the near term and to diversify the economic
base of the region in the long term. (Draft Environmental Assessment,
DOE/EA-1113, August 1995)

With this opening paragraph of the draft Environmental Assessment
DOE clearly states that it plans to lease part of the 35,000 acre
(142 km2) Oak Ridge Reservation for economic development. The
ED-1 land parcel is one of several candidate areas (the ED-1
parcel is labeled Par. 1 in the figure above). Before this parcel can
be developed, however, the National Environmental Protection Act
(NEPA) requires that the DOE prepare an environmental assessment
to address the potential effects of the proposed development on
the environment, and the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA) requires a review of
the site for existing hazards. Although there is no record that
the ED-1 land parcel was ever used for waste storage or disposal,
the entire Oak Ridge Reservation is a Superfund site. Consequently,
as part of the CERCLA review DOE asked the Environmental Restoration
(ER) Remote Sensing Program to examine all relevant remote sensing
data.

Airborne Geophysical Data

In 1992-93, the ER Remote Sensing Program collected aerial geophysical
data over the entire Oak Ridge Reservation, including measurements
of the magnetic total field, vertical magnetic gradient, and multi-frequency
electromagnetic induction
(Doll et al., 1993 ).
In this paper,
we describe the results of our review of the airborne geophysical
data for the ED-1 land parcel, our follow-up field investigations,
and the consequences for the environmental assessment of the ED-1
land parcel. Only the geophysical findings are discussed; the
results of the wetlands, biodiversity, ambient noise, and other
surveys conducted as part of the environmental assessment are
not discussed in the paper.

DESCRIPTION AND HISTORY OF THE ED-1 LAND PARCEL

A patchwork of small farms and coal mining communities blanketed
the area which now comprises the ORR until 1942, when the federal
government chose this area to be the site of one of the Manhattan
Project production facilities. To isolate the Oak Ridge facilities,
the government annexed over 35,000 acres (142 km2) of land. This
land has remained under federal control for more than fifty years.
Presently, the ORR includes three main facilities: the K-25 Gaseous
Diffusion Plant, the Y-12 Plant, and the X-10 plant (commonly
known as Oak Ridge National Laboratory). Also within the ORR are
a number of hazardous waste burial grounds. The majority of the
ORR, however, is farm land that has reverted to forest. Historical
aerial photographs taken prior to 1942 show a number of small
farms on the ED-1 land parcel, but the area is now largely covered
with neat rows of pine trees planted as part of a tree farming
study.

The local geology is complex. East Tennessee is part of the Appalachian
thrust and fold belt; consequently, the regional topography exhibits
a series of nearly parallel valleys and ridges. The ED-1 land
parcel consists of roughly 1000 acres (400 hectares) of land located
east of the K-25 plant and west of the City of Oak Ridge in a
valley bounded by Pine Ridge to the south and Black Oak Ridge
to the north. East Fork Poplar Creek flows from east to west across
the center of the parcel, and drains into the Clinch River near
the K-25 plant. Across most of the ORR the local geology consists
of Cambrian to Mississippian age shale and limestone beds, which
strike parallel to the valleys, and dip to the south at average
45 degrees. The ED-1 land parcel is underlain by the predominately
carbonate Chickamauga Group, and includes part of the structurally
complex East Fork Syncline. Typically, the shales are highly fractured,
the carbonates riddled with solution cavities, creating a complex
hydrogeologic setting (Hatcher et al., 1992).

DISCUSSION OF THE AERIAL DATA FOR ED-1 AND THE SURROUNDING
AREA

Background

In 1992-3, reconnaissance airborne geophysical data was collected
for the entire ORR, with an average line spacing of 50 m. The
average electromagnetic sensor flight height was 30 m above the
ground surface; the magnetometers where on the tow cable 20 m
higher, for an average flight height of 50 m. Based on analysis
of the reconnaissance data, we selected a number of areas for
high-resolution follow-up airborne surveys; these areas were flown
in the spring of 1994. Fortuitously, the ED-1 land parcel is contained
within one of the high-resolution survey areas, so our discussion
is based on the high-resolution data.

The high-resolution surveys were flown using a small bird developed
for this survey by the contractor, Geonex Aerodat. The system
features a five-frequency, horizontal coplanar loop, electromagnetic
induction system operating at 7426, 22278, 37130, 51982, and 66834
Hz, with a transmitter-receiver coil separation of 3.76 m. Located
in the tail of the bird were a pair of high-sensitivity cesium-vapor
magnetometers separated by 1 m to form a vertical magnetic gradiometer.
Position and altitude information were provided by a differential
global positioning system receiver, and both laser and radar altimeters,
located within the bird. The high-resolution survey line spacing
was reduced to about 40 m, but because the area is wooded, the
altitude was again about 30 m. Although the bird altitude was
roughly the same for both the reconnaissance and high resolution
surveys in wooded areas, the magnetometers were 20 m closer to
the ground in the high resolution survey because the magnetometers
were mounted in the tail of the specially designed high resolution
bird, rather than on the tow cable above the bird
(Beard et al., 1995).

Vertical Magnetic Gradient

Vertical magnetic gradient data is the key to determining whether
or not there is undocumented buried waste on the ED-1 land parcel,
because most waste is buried in ferrous-metal waste canisters.
The figure above shows the vertical gradient
data plotted over a range
of -2.8 to 5.0 nT/m. The color in this figure equivalent to the
range of -0.015 to 0.015 nT/m has been rendered transparent for
two reasons: first, this makes the small localized anomalies more
visible by eliminating the background of nearly the same color;
second, this range approximately coincides to the electronic noise
of the magnetometers, so the remaining anomalies are less likely
to be the result of instrument noise. The data are dominated by
the magnetic response from White Wing Scrap Yard (a waste area
south of the ED-1 land parcel), a highway overpass, and several
power lines, but numerous unidentified small anomalies are present
within the ED-1 land parcel.

Because the goal was not simply to locate major anomalies, but
to determine whether or not the parcel is "clean," small
anomalies could not be ignored. This makes it vitally important
to distinguish between significant anomalies and noise. At issue
were instrument noise, noise induced by flight height variations,
and geologic noise. We have accumulated considerable evidence
that geologic noise can be an order of magnitude greater than
instrument noise at several locations on the ORR. There is evidence
that this geologic noise is created by accumulations of maghemite
in the soil (Doll et al., 1995, Kopp and Lee, 1985).

In the case of the ED-1 land parcel we had less than week to respond
to DOE's request for information, so rather than systematically
catalog all of the anomalies we focused on only the strongest
anomalies within, or close to, the parcel boundary. Each small,
localized anomaly was converted to an equivalent dipole source
strength in kilograms of iron (Breiner, 1973).
All anomalies estimated
at 1000 kg or more were field checked, along with some smaller
anomalies. The results are discussed below.

ANOMALY A: WHITE WING SCRAP YARD

White Wing Scrap Yard is a known waste area. It was used for surface
storage of scrap materials by all three of the Oak Ridge plants,
and although is was not intended as a burial ground, many objects
are buried there. White Wing Scrap Yard is important for the characterization
of the ED-1 land parcel because a tributary of West Bear Creek
flows through part of the scrap yard into the ED-1 land parcel
and empties into East Fork Poplar Creek, providing a potential
for off-site contamination of the ED-1 land parcel. Furthermore,
piezometer wells drilled at White Wing Scrap Yard encountered
large solution cavities. Water level measurements revealed a closed
water table, with flow directed toward the center of White Wing
Scrap Yard, rather than the nearby creek. This suggests a karst
related subterranean exit path exists, probably draining in the
direction of the ED-1 land parcel.

ANOMALY B: HIGHWAY OVERPASS

The largest magnetic anomaly in the survey area corresponds to
a highway overpass at the junction of state highways 95 and 58.
Overlaying the magnetic data on a site map or aerial photograph
of the site makes it easy to identify the effects of large structures
and power lines.

ANOMALY C: SMALL BRIDGES

Four of the anomalies within the ED-1 land parcel were created
by bridges. Three are along a gravel, two-lane perimeter road,
and one was an abandoned bridge crossing East Fork Poplar Creek
in the eastern portion of the land parcel.

ANOMALY D: LOGGING TRUCKS

In our initial follow-up visit to the field, we failed to find
the sources a number of magnetic anomalies along the roads, but
not near any bridges or power lines. We reviewed the airborne
video tapes collected when the survey was be flown and discovered
that these anomalies corresponded to logging trucks.
Although the forests on the ORR are not harvested for timber,
many of the pine forests planted in the ED-1 land parcel died
as the result of a pine beetle infestation, and were being removed
at the time of the airborne geophysical survey. This example illustrates
the importance of collecting high-quality video data during airborne
geophysical surveys of environmental sites as "transient
anomalies" could otherwise be mistaken for buried objects.

ANOMALY E: LAMBERT'S QUARRY

Lambert's quarry is located just outside the northeast corner
of the ED-1 land parcel. It is not clear whether the
magnetic anomalies seen in this area stem only from metal objects
around the quarry, or from within the quarry itself. In our field
investigations we found several possible sources around the quarry:
a pile of concrete rubble containing steel reinforcing rods (rebar),
a concrete pierthat may have been part of a conveyor belt leading
to the quarry, some corroded steel cylinders probably
used to hold compressed gas, and a single 55-gallon
drum half buried in a mound of dirt.
Old truck tires and other debris were visible
beneath the quarry waters
near the shore, evidence that waste (not necessarily hazardous)
was dumped in the quarry.

ANOMALY F: UNIDENTIFIED

We did not have time to follow-up on all of the magnetic anomalies
within the ED-1 land parcel, and their sources remain unidentified.
In a few cases we were unable to locate the source. At those sites
a ground based magnetic survey would be required to confirm that
the aerial magnetic anomaly was not the result of geologic or
system noise, but this was outside the limited scope of our investigation.
Note that this land is densely overgrown with trees, shrubs, and
brambles. Even guided by the aerial magnetic
data, the metallic debris that we discovered near Lambert's quarry
was difficult to find.

Apparent Resistivity

The figure above shows the apparent resistivity determined by a inversion
of the 7 KHz electromagnetic data assuming a homogeneous halfspace.
The response is a combination of the local geology and cultural
features. The shaley carbonate formations are relatively conductive;
the purer carbonate formations are relatively resistive. Near
the north-northwest boundary of the ED-1 land parcel is a large
conductive anomaly within the Mascot dolomite and the Pond Spring
limestone. This anomaly coincides with a large sinkhole that we
believe has partially filled with clay. There is an east-west
conductive trend connecting this sinkhole to Lambert's quarry.
This trend, coupled with the presence of other smaller sinkholes
in the area, supports the hypothesis of a groundwater conduit
running along strike almost the entire length of the northern
boundary of the ED-1 land parcel.

The resistivity data appears to be most sensitive to the soil
and bedrock geology (note the geologic contacts overlain in gray).
The highway overpass and White Wing Scrap Yard,
noted in our discussion of the magnetic data, are also visible
in the resistivity data. However, none of the smaller magnetic
anomalies have corresponding conductive anomalies.

CONCLUSIONS

Based on the aerial magnetic data, we conclude that there are
no major burial grounds on the ED-1 land parcel. White Wing Scrap
Yard is probably the smallest of the known waste storage areas
on the ORR, yet it produces a much larger magnetic anomaly than
any within the ED-1 land parcel. The metallic debris surrounding
Lambert's quarry deserves further investigation. Abandoned quarries
are notorious for attracting illegal dumping, and Lambert's quarry
is both accessible and remote. The single empty, corroded, 55-gallon
drum that we discovered near the quarry probably does not represent
a health hazard, but is probably left over from quarry operations.
Nevertheless, a ground-based magnetic survey would be reasonable
to determine whether any buried drums are present. As a consequence
of the short time-table imposed by DOE, we did not have time to
investigate the remaining small magnetic anomalies, although all
but one of the anomalies equivalent to 1000 kg or more of iron
have been identified.

The resistivities calculated from the electromagnetic induction
data were more sensitive to the local soil and geology than to
metallic objects. These data support what other investigators
reported based on earlier field studies: that the carbonate formations
within the ED-1 land parcel are highly karstified, making this
a sensitive hydrogeologic area. If a karst conduit exists, contaminants
introduced at this site could migrate to the Clinch River (a public
waterway) much more rapidly than would be predicted by non-karst
groundwater flow models.

Apparently, the greatest contamination concern for the ED-1 land
parcel arises not from materials disposed within the parcel, but
from contaminants migrating into the parcel from other sources.
That East Fork Poplar Creek carries mercury contamination from
the Y-12 plant into the ED-1 land parcel was known before the
lease was proposed. Consequently, the developers plan to leave
the East Fork Poplar Creek floodplains untouched. West Bear Creek,
a tributary of East Fork Poplar Creek, may carry contamination
from White Wing Scrap Yard, but because there are no groundwater
monitoring or stream sediment data available, this remains conjecture.
Finally, we note that Lambert's quarry, which is located just
outside the ED-1 boundary, may have been used for dumping,
and deserves further investigation. The quarry is spring fed,
and may be connected to a karst conduit system running east-west
across the ED-1 land parcel.

In summary, this case study shows what we believe to be the principal
utility of airborne geophysical data at large hazardous waste
sites: the ability to screen large areas of land for undocumented
waste disposal sites. Surveying this densely vegetated 1000 acres
(400 hectare) site on foot would have been slow and expensive.
Although no geophysical technique can be used to certify that
an area is absolutely free of contamination, airborne geophysics
shows promise as a tool to screen large areas for buried waste,
and thus provide relatively inexpensive protection against major
liability in the future.

ACKNOWLEDGEMENTS

This work was performed at Oak Ridge National Laboratory operated
by Lockheed Martin Energy Systems, Inc., for the U. S. Department
of Energy under contract DE-AC05-84OR21400. Financial support
was sponsored by the Oak Ridge Environmental Remote Sensing Program.

Kopp, O.C. and Lee, S.Y., (1985), An unusual occurrence
of maghemite in soils developed on dolostones of the Knox Group,
Oak Ridge, TN, Proceedings of the International Clay Conference,
Denver, p 205-211.

Review of Parcel ED-1, Under Section 120 (h) of the Comprehensive
Environmental Response, Compensation, and Liability Act and Section
XLIII of the Federal Facility Agreement, August 1995, 13pp.